The transition region above sunspots
Tian et al. Geosci. Lett.
The transition region above sunspots
Hui Tian 0
Tanmoy Samanta 0
Jingwen Zhang 0
0 School of Earth and Space Sciences, Peking University , Beijing 100871 , China
Over decades, sunspots and their fine structures have been studied in detail at the photospheric level with different ground-based telescopes, as the surface of the Sun primarily emits light in the visible wavelengths. For a very long period, the upper atmosphere above the sunspot regions, especially the transition region (TR) above sunspots where the plasma emits light in the far ultraviolet (FUV) and extreme ultraviolet (EUV), has been poorly understood. In the past decades after the development of space instrumentations, FUV and EUV observations have uncovered many secrets of the TR above sunspots. In this paper, we present a brief review of research results about the TR structures and dynamics obtained through imaging and spectroscopic observations of sunspots in the past ~ 20 years. Though these observations have gathered remarkable and detailed information and greatly improved our understanding of the TR above the sunspots, paradoxically, they leave us with many new questions which should be answered in the future.
Sunspots; Solar transition region; Chromosphere
Introduction
Sunspots are the darkest regions with the strongest
magnetic fields and lowest temperatures on the surface of
the Sun. Though observed by humanity for thousands of
years, detailed investigation of the long-term evolution of
sunspots started only 400 years ago, when Galileo
Galilei invented the astronomical telescope. The number of
sunspots strongly varies on a timescale of about 11 years,
which is known as the sunspot cycle or solar cycle.
Correspondingly, the occurring frequency of large-scale solar
activities such as solar flares and coronal mass ejections
also changes with such an 11-year cycle. As a result, the
Earth’s magnetosphere and ionosphere are disturbed by
the same cycle. The total electromagnetic radiation of the
Sun is also closely related to the 11-year sunspot cycle.
For instance, the extreme ultraviolet (EUV) and far
ultraviolet (FUV) emission of the Sun varies significantly over
a solar cycle.
Modern high-resolution observations by
large-aperture telescopes have revealed detailed substructures
and short-term dynamics of sunspots. A sunspot often
consists of a dark core termed umbra and a less dark
penumbra surrounding the umbra. Numerous umbral
dots are often found inside the dark core, while the most
prominent structures in the penumbra are the so-called
penumbral filaments. Systematic outward flows, termed
Evershed flows
(Evershed 1909)
, are usually present in
the penumbra. Some sunspots also have light bridges,
which are bright lane-like structures dividing the umbra
into two or more parts. It is believed that umbral dots,
penumbral filaments, and light bridges are formed as
a result of vigorous convective motions. For a detailed
description of these substructures and dynamics, we refer
to the comprehensive review by
Solanki (2003)
. Some of
these substructures and dynamics have been well
reproduced through radiative magnetohydrodynamic (MHD)
simulations
(see a review by Rempel and Schlichenmaier
2011)
.
Most studies of sunspots are focused on the
photospheric structures and dynamics. For a very long period,
the upper atmosphere in sunspot regions, especially
the transition region (TR) above the sunspots, has been
poorly understood. This is mainly due to the fact that
there were only very limited numbers of TR
observations in the sunspot regions. By definition, the TR is the
interface region between the chromosphere and corona,
where the temperature increases from roughly 2 × 104 to
8 × 105 K
(e.g., Tian 2017)
. TR probing relies mainly on
imaging and spectroscopic observations of the EUV and
FUV emission, primarily in the spectral range of 400–
1600 Å. In the past ~ 20 years, observations from mainly
three EUV/FUV spectrographs have greatly improved
our understanding of the TR. The first two instruments
are the Solar Ultraviolet Measurements of Emitted
Radiation
(SUMER, Wilhelm et al. 1995)
and Coronal
Diagnostic Spectrometer
(CDS, Harrison et al. 1995)
onboard the
Solar and Heliospheric Observatory (SOHO) launched
in late 1995. The SUMER instrument has a very broad
wavelength coverage, from roughly 660 to 1610 Å.
Hundreds of strong TR lines have been observed by SUMER.
The spatial resolution of SUMER observations is about
2′′–3′′. Unfortunately, most SUMER observations were
performed in the first few years after the launch of
SOHO, when the solar activity was relatively low. As
a result, there are limited numbers of sunspot
observations by SUMER. For more than one solar cycle,
spectra of some strong TR lines have been acquired in many
CDS observations. However, the CDS instrument was
mainly designed for coronal studies and the spatial
resolution of the data is of much poorer quality than that of
SUMER. (...truncated)